The invention relates to a magnetic tape drive for a recording or playback apparatus housing; a capstan for driving the magnetic tape with an accurately defined speed; a tape guide member around which the magnetic tape coming from the capstan is passed under tension; a plurality of magnetic heads for cooperation with the magnetic tape; and a control device for maintaining the tension in a part of the magnetic tape disposed upstream from said heads constant within certain limits; and in particular to such a drive for an apparatus whose heads contact the tape along a tension-controlled loop.
The use of a closed or tension-controlled loop in magnetic tape drive arrangements serves mainly to obtain an accurately defined speed of the magnetic tape, at the location where the tape cooperates with the magnetic heads. In a known apparatus utilizing a tension-controlled loop formed with a single capstan (U.S. Pat. No. 2,948,488) the magnetic tape is passed from the supply reel to a fixed tape guide roller amounted on the deck of the apparatus, towards a subsequent tape guide roller which is mounted on a lever which is pivotable about its end. The position of this lever is variable, so that fluctuations in the transport speed of the magnetic tape can be taken up in such a way that the tension of the magnetic tape can remain constant. Directly before the take-up reel the magnetic tape is guided in a similar way. The tension in the magnetic tape immediately upstream and immediately downstream from the closed loop is determined by the drive of the capstan, the supply reel and the take-up reel. The objective is to maintain the tension in the magnetic tape in the closed loop as constant as possible and to isolate it as far as possible from tape-tension fluctuations outside the closed loop. It is important to apply such steps that it is ensured that the tape tension always has a positive value anywhere in the closed loop, because otherwise undesired loops may be formed locally in the closed loop.
Tests have revealed that in tension-controlled-loop magnetic tape drive arrangements the tape tension in the loop directly upstream from the capstan, that is at one end of the loop, is directly proportional to the tape tension directly outside the loop at the other end. As the magnetic tape in the loop runs over a number of surfaces, so that frictional forces are exerted on the magnetic tape, there is a risk that the tape tension in the tension-controlled loop directly downstream from the capstan becomes too low and looping occurs.
Therefore, it is essential to give the tape tension before the loop such a constant value that looping of the magnetic tape in the loop is prevented at all times. Furthermore, it is of importance that the tape tensions are minimal. In view of these two factors it will be evident that the tape tension in the loop is to be kept both as low and as constant as possible. Only slight variations of the tape tension in the loop are permissible. In practice it is found that the coefficients of friction of the various elements which cooperate with the magnetic tape in the loop, such as the magnetic heads and the said tape guide member, are variable under the influence of the ambient temperature and the ambient humidity. If looping of the magnetic tape is to be prevented at all times, the tape tension should be optimized so that it still suffices under the most unfavourable conditions.